DISCOVERY 



177 



devastated. Such exceptional instances explain the 

 occurrence in the Pacific of reefs whose broad surface 

 is stre\\Ti only with dead corals, with a few colonics 

 beginning to work in again from the edge. 



There is, apart from this, a marked difference to 

 be traced between the oceanic shores of an island 

 and those within a harbour or estuary where muddy 

 invasions may be usually expected in the wet season. 

 In the latter position the beautiful Acroporas and the 

 encrusting Lithothamnion are entirely absent. The 

 massive brain corals and the flabby Alc\-onarian corals 

 (relatedto our "Dead Men's Fingers ") are the dominant 

 forms of such impoverished reefs, and these corals 

 are provided with an efficient mechanism to get rid 

 of the suspended mud which threatens to choke 

 them. 



Many interesting cases of distribution according to 

 environment are to be found among the general 

 invertebrate fauna of a coral reef. The wave-pounded 

 edge would seem to offer a singularly ineligible shelter 

 for any lodger, and yet characteristic forms are found 

 there. The sea-urchin, Heterocentrotus, is widespread 

 in the tropics, and a familiar but puzzling form in 

 collections. Seen in its natural habitat, the meaning of 

 its huge mace-like spines becomes apparent. They 

 are pushed into tiny hollows and fissures of the coral 

 blocks, and with the aid of the sucking " tube feet " 

 are quite adequate to maintain the sea-urchin in posi- 

 tion against a heavy wave and its backwash. Of 



photograph by W. H. LongUy. 



Fig. 3.— schooi, of pork fishes, massive coral and sea- 

 urchins icextrechlxus), portugas. 



entirely different appearance is the equallv characteristic 

 long-spined urchin, Centrechinns, which is found only 

 at moderate depths or in the quiet pools of the reef 

 (Fig. 3). The variations in the form of the spines 



of urchins can be thus roughly compared with those 

 of the corals themselves. 



Of the active population of a coral reef, the fishes 

 are without doubt the most interesting. Even the 



Fig. 4. 



Photograph iy If. H. Longlcy. 

 -SCHOOI, OF NEOM.EXIS GRISEUS, PORTUGAS. 



least thoughtful and speculative traveller is always 

 e.xcited to admiration by them, and indeed they seem 

 such creatures of romance that it is shameful to subject 

 them and their doings to scientific analysis. But coral 

 reef fishes form a biological complex of the greatest 

 interest. The credit of much careful pioneer work 

 in the West Indies and the Pacific is due to Professor 

 Longley, of Baltimore. He has gathered together a 

 mass of information concerning their habits, their 

 coloration and patterns, their conspicuousness against 

 natural backgrounds, their food and many other 

 matters. 



A permanent record of many of these points has been 

 secured by his remarkable underwater photographs, 

 t xamples of which are shown amongst the illustrations to 

 this article. A diving-hood worn over the shoulders 

 and leaving the hands free to manage a camera enables 

 Longley to work under water in the shallow reef chan- 

 nels for hours at a time. He is able to move about 

 slowly to the limit of the 100 feet of rubber piping 

 which connect him with the boat. Though a diver 

 cuts a strange figure according to our terrestrial ideas, 

 he does not seem so strikingly out of place in a sub- 

 marine landscape. The fishes, which will fly from the 

 shadow of the boat overhead, yet show little or no fear 

 of this uncouth monster groping his way along, and 

 do not suspend their pressing business for him. 



The camera used for these photographs is a 5 x 4 

 inch " Autograflex " enclosed in a watertight metal 

 container. It is a bulky object to carry and takes 

 immense patience to manoeuvre into position for an 

 exposure, even when the fish are complacent " sitters." 



